Process for producing metal foam and metal body produced using this process

ABSTRACT

The invention relates to a process for producing metal foam and to a metal body produced using this process.  
     The object is achieved by a process for producing metal foam by adding a blowing agent to a metal melt, wherein the metal melt is introduced into the die cavity of a metal die-casting machine and is foamed using a blowing agent which releases gases and is solid at room temperature.

RELATED APPLICATION

[0001] This application claims priority to German application nos. 10104 339.2 and 101 04 340.6, filed Feb. 1, 2001, both herein incorporatedby reference.

BACKGROUND OF TIE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to a process for producing a metal foam ina metal die-casting machine, in particular for a producing metal foamfrom aluminum materials. This invention further provides for a metalbody produced using this process; for example, a component made from analuminum material of this type.

[0004] 2. Discussion of the Related Art

[0005] It is known to produce components from foamed metallic materials.These components are distinguished by inter alia a lightweightstructure, rigidity, compressive strength improved mechanical andacoustic damping. By way of example, cast cores of aluminum foam aresurrounded with an aluminum material by casting or are inserted asshaped parts into a component The sheath and core or shaped part areproduced separately and are then joined to one another. In addition tothe high manufacturing outlay, this also leads to a low manufacturingquality.

[0006] The basis of foamable semifinished aluminum products is atomizedaluminum powder to which a blowing agent is added. By way of example,according to DE-A-197 44 300, a body which has been pressed from apowder mixture is heated, in a heatable, closed vessel, to temperatureswhich are higher than the decomposition temperature of the blowing agentand/or the melting temperature of the metal.

[0007] The powder is compressed and the shaped part produced in this wayis inserted into the area of a component, which is to be foamed. Thepowder is foamed by being heated to up to 650° C. In the process, the“sheath” may be subject to unacceptable deformation or the foamingoperation may take place non-uniformly.

[0008] Sintering of metallic hollow beads or infiltration of metal meltsinto cores or filler bodies, which are removed after solidification ofthe melt, is also possible.

[0009] According to a process described in JP-A-03017236, metallicarticles with cavities are produced by dissolving gases in a metal meltand initiating the foaming operation by suddenly reducing the pressure.The foam is stabilized by cooling of the melt.

[0010] According to the teaching given in JP-A-09241780, metallic foamis obtained with the controlled release of blowing gases as a result ofa metal initially being melted at temperatures which lie below thedecomposition temperature of the blowing agent used. Subsequentdispersion of the blowing agent in the molten metal and heating of thematrix to above the temperature which is then required to releaseblowing gases leads to a metal foam being formed.

[0011] W. Thiele: Füllstoffhaltiger Alumumschwamm—ein kompressiblerGusswerkstoff zur Absorption von Stoβenergie, [Filler-containingaluminum sponge—a compressible cast material for absorption of impactenergy], in: Metall, 28, 1974, Vol. 1, pp. 39 to 42, describes theproduction of foamed aluminum. The desired cavities are predetermined interms of size, shape and position in the form of a loose bed of readilycompressible, inorganic light materials, such as for example expandedclay minerals, expanded clay, glass foam beads or hollow corundum beads,etc. The bed of light material is introduced into a die. The spaceswhich remain in the bed are filled with metal. The aluminum spongeobtained in this way has relatively poor mechanical qualities andcontains the materials of the bed.

[0012] DE-B-11 64 103 describes a process for producing metal foambodies. In this process, a solid material which, when heated, decomposesto form gases, is mixed with a molten metal in such a manner that thesolid material is wetted by the metal. By way of example, pulverulenttitanium hydride is added to a molten alloy of aluminum and magnesium ata temperature of 600° C. The closed foam formed in this way is then castinto a die, where it can cool and solidify. In this case too, it isclearly not a closed system, but rather an open system which is used.

[0013] GB-A 892934 describes the production of complex structures withfoamed metal core and closed, nonporous surface, the execution of thedescribed process being dependent on the introduction of themetal/blowing agent mixture into the die before the start of the foamingprocess.

[0014] DE-C 198 32 794 describes a process for producing a hollowprofiled section which is filled with metal foam. This process comprisesthe steps of extruding the hollow profiled section from a sheathingmaterial using an extruder that has an extrusion die with a die part anda mandrel, supplying the metal foam from a foam material to the hollowprofiled section through a feed duct, which is formed in the mandrel.

[0015] The casting of metal parts with lost foam is already known inaccordance with EP-B 0 461 052. WO 92/21457 describes the production ofaluminum foam in such a manner that gas is blown in under the surface ofa molten metal with abrasives being used as stabilizers.

[0016] EP-B 0 666 784 describes a process for the shape casting of ametal foam which is stabilized by means of particles, in particular analuminum alloy, by heating a composite of a metal matrix and finelydivided solid stabilizer particles above the solidus temperature of themetal matrix and discharging gas bubbles into the molten metal compositebelow the surface thereof to form a stabilized liquid foam on thesurface of the molten metal composite. The characterizing feature isshape casting of the metal foam by the stabilized, liquid foam beingpressed into a mold, using a pressure which is just sufficient for theliquid foam to adopt the shape of the mold, without the cells of thefoam being significantly compressed, and then cooling and solidifyingthe foam, in order to obtain a shaped object. The foam is in this casepressed into the mold by means of a moveable plate. A first moveableplate presses the liquid foam into the mold and a smooth surface isformed on the shaped foam object. In order to form smooth inner surfaceson the foam object, a second moveable plate is pressed into the foaminside the mold. However, the shaping may also take place by means ofrollers.

[0017] A further process for making castings from metal foam isdescribed in EP-A 0 804 982. In this case, the foaming takes place in aheatable chamber outside a casting mold, the volume of the powdermetallurgy starting material introduced into the chamber for the metalfoam, in its phase in which it has been foamed with the entire foamingcapacity, substantially corresponding to the volume of a filled mold.All the metal foam in the chamber is pressed into the mold, in whichfoaming with the remaining foaming capacity is continued until the moldhas been completely filled. The mold is a sand or ceramic mold, themetal foam is inserted into the chamber as a semifinished product and isonly pressed into the mold, for example by means of a piston, after theinitial foaming. When the foam is being pressed into the mold, it issheared. The mold is not filled with a foam with a structure which isdeliberately inhomogeneous.

[0018] DE-A 19 501 508 discloses a process for producing a hollowprofiled section of reduced weight and increased rigidity, for example acomponent for the chassis of a motor vehicle. It comprises die-castaluminum, in the cavities of which there is a core of aluminum foam. Theintegrated foam core is produced by powder metallurgy and is then fixedto the inner wall of a casting die and surrounded with metal bydie-casting.

[0019] The dissolving or blowing of blowing gases into metal melts isnot suitable for the production of near net shape components, since asystem comprising melt with occluded gas bubbles is not stable for asufficient time for it to be processed in shaping dies.

OBJECT OF THE INVENTION

[0020] Therefore, it is an object of the invention to provide a simplemethod for producing metal foam which is suitable for mass productionand allows near net shape metal bodies to be produced with littleoutlay.

SUMMARY OF THE INVENTION

[0021] The solution to the above object consists, in a first embodiment,in a process for producing metal foam by adding a blowing agent to ametal melt, wherein the metal melt is introduced into the die cavity ofa metal die-casting machine and is foamed using a blowing agent whichreleases gases and is solid at room temperature.

DETAILED DISCUSSION OF THE INVENTION

[0022] Surprisingly, it has been discovered that, in particular,light-metal foams, for example comprising aluminum or aluminum alloys,can be produced as integral foams, i.e. with a closed outer skin, in atargeted manner as a gradient material and near net shape in one step bymeans of a casting operation, using a commercially available die-castingmachine, based on the use of solid, gas-releasing blowing agents, forexample a metal hydride, in particular a light metal hydride.

[0023] During die-casting in the sense of the present invention, liquidor pasty metal is forced at high pressure into a die which forms the diecavity. In the case of the hot-chamber process, the metal is injecteddirectly from the melting pot at up to approx. 10⁷ Pa into the die. Inthe cold-chamber process, which is preferred according to the invention,for example for materials comprising Al alloys and Mg alloys, the moltenmaterial is first pressed into a cold intermediate chamber and, fromthere, is pressed into the die at more than 10⁸ Pa. The castingperformance of the hot-chamber process is higher, but so is the wear tothe installation. The benefits of die-casting are the good strength ofthe material, the clean surface, the high dimensional accuracy, the lowwall thicknesses required, the possibility of forming castings ofcomplex shape and the high working rate. These advantages can be furtherimproved by subatmospheric pressure (vacuum) in the die. Commerciallyavailable, real-time controlled die-casting machines can advantageouslybe used in this process.

[0024] In a preferred embodiment of the present invention, the metalsare selected from nonferrous metals and base metals, in particularselected from magnesium, calcium, aluminum, silicon, titanium or zincand the alloys thereof On the other hand, ferrous metals and preciousmetals can also be foamed with the aid of the present invention. Wherethe present invention uses the term alloy, this term is to be understoodas meaning that the alloy contains at least about 30% by weight of themetal mentioned.

[0025] The process sequence, which is preferred according to theinvention, comprises the step of introducing the required volume ofmetal melt into the shot sleeve or chamber and transferring it into adie cavity with the blowing agent being added to the metal melt In apreferred embodiment, metal melt and blowing agent are brought togetherin the die cavity with the die being filled or underfilled by a definedvolume of the melt/blowing agent mixture.

[0026] In a further preferred embodiment, the blowing agent is broughtinto contact with the metal melt not directly in the die cavity, butrather in a shot sleeve or chamber, and the mixture is then introducedinto the die cavity.

[0027] The introduction of the blowing agent into the shot sleeve orchamber, on the one hand, and/or the die cavity, on the other hand, maytake place before, during and/or after the introduction of the metalmelt into the chamber in question. There is no need for a special mixingoperation when the metal melt and blowing agent are brought together.Rather, in the process according to the invention, mixing is effected bythe introduction of the metal melt into the die chamber, which in thedie-casting machine takes place at a high velocity, irrespective of thetime at which the blowing agent is added to the metal melt.

[0028] However, for the present invention it is important for thefoaming caused by the release of gases from the blowing agentsubstantially to take place only in the die cavity.

[0029] The foaming itself takes place in the die cavity, whichrepresents a closed die. However, it may have risers for venting, as iscustomary in die-casting or the like. Then, the substantially foamedmetal body is ejected.

[0030] In a further configuration, the blowing agent is added to themetal melt directly in the shot sleeve or chamber or in the die cavity,with integrally foamed metal bodies being produced in each case in oneworking step. These bodies have a smooth surface, the formation of whichis readily reproducible. Different wall thicknesses are easy toestablish on account of the possibility of spray filling. The walls areclosed on all sides, are clean, nonporous and homogeneous. Furthermachinery is not generally required. Toward the inside, the metal bodiesproduced are increasingly porous and have a density gradient.

[0031] In terms of its decomposition temperature, the blowing agentshould be adapted to the melting temperature of the casting material(metal melt). The decomposition must only commence at over 100° C. andshould be no more than approximately 150° C. higher than the meltingtemperature.

[0032] The quantity of blowing agent to be used depends on the requiredconditions. Within the context of the present invention, it isparticularly preferable for the blowing agent to be used in a quantityof from about 0.1 to about 10% by weight, in particular about 0.2 toabout 1% by weight, based on the metal melt.

[0033] Blowing agents which release gases and are solid at roomtemperature include, in particular, light-metal hydrides, such asmagnesium hydride. In the context of the present invention,autocatalytically produced magnesium hydride, which is marketed, forexample, under the name TEGO Magnan by the applicant, is particularlypreferred within the context of the present invention. However, titaniumhydride, carbonates, hydrates and/or volatile substances, which havealready been used in the prior art to foam metals, can also be used inthe same way.

[0034] Based on solid material, the proportion of metal in the metalbody produced may be in the range from about 5 to about 95% by volume or% by weight, depending on the volume or thickness of the metal body, alower volume to area ratio meaning higher degrees of filling.

[0035] The invention is described in more detail below in an exemplaryembodiment

EXAMPLES

[0036] A vehicle component made from an aluminum material was to beproduced as an integrally foamed metal body using a commerciallyavailable die-casting machine, for example a die-casting machinebelonging to the Evolution series from Bühler Druckguss A G. For thispurpose, a shot chamber of the die-casting machine was filled with acorresponding quantity of metal melt. Magnesium hydride in powder formwas added to the liquid metal as the blowing agent in the closed shotchamber. Almost at the same time, the mixture of blowing agent and metalmelt began to be forced quickly into the die cavity. The die cavity wasunderfilled with a defined volume. The turbulence produced led tointimate mixing in the die cavity and to foaming. The metal solidifiedon the die walls and formed a nonporous, homogeneous wall of the metalbody, it was possible to set both the wall thicknesses and the porosityand its gradient by varying process parameters.

[0037] The “shot” took place before the formation of the foam; thefoaming process took place in situ in the die cavity. The melt wasquickly foamed into the cold die. The component had a mass of onlyapproximately 40% compared to conventional die castings.

[0038] Although the metal body produced according to the example from analuminum material had a lower density than magnesium, its torsionalrigidity was approximately the same.

[0039] The above description is intended to be illustrative and notlimiting. Various changes and modifications in the embodiment describedherein may occur to those skilled in the art. Those changes can be madewithout departing from the scope and spirit of the invention.

What is claimed is:
 1. A process for producing a metal foam in adie-casting machine comprising a sleeve or short chamber and a diecavity which comprises foaming a mixture comprising a metal melt and ablowing agent in the die cavity, whereby the blowing agent is solid atroom temperature.
 2. The process according to claim 1, wherein themixture comprising the metal melt and the blowing agent is formed byintroducing the metal melt and the blowing agent into the die cavity atthe same time.
 3. The process according to claim 1, wherein the mixturecomprising the metal melt and the blowing agent is formed beforeintroducing the mixture to the die cavity.
 4. The process according toclaim 3, wherein the mixture comprising the metal melt and the blowingagent is formed in the sleeve or shot chamber and is then introduced tothe die cavity.
 5. The process according to claim 1, whererin themixture comprising the metal melt and the blowing agent is formed byintroducing first the metal melt into the die cavity and then adding theblowing agent.
 6. The process according to claim 1, wherein the mixturecomprising the metal melt and the blowing agent is formed by introducingthe blowing agent into the die cavity and then adding the metal melt. 7.The process according to claim 1, wherein the die cavity is filled withthe mixture comprising the metal melt and the blowing agent beforefoaming said mixture.
 8. The process according to claim 7, wherein thedie cavity is not heated.
 9. The process according to claim 1, whereinthe die cavity is underfilled by a defined volume.
 10. The processaccording to claim 9, where in the die cavity is heated.
 11. The processaccording to claim 1, wherein the metal melt is a melt of a light metal.12. The process according to claim 1, wherein the metal is aluminum oran aluminum alloy.
 13. The process according to claim 1, wherein themetal is Mg or a Mg alloy.
 14. The process according to claim 1, whereinthe process is a cold-chamber process.
 15. The process according toclaim 1, wherein the process is a hot-chamber process.
 16. The processaccording to claim 1, wherein the blowing agent is a metal hydride. 17.The process according to claim 16, wherein the metal hydride is alight-metal hydride.
 18. The process according to claim 17, wherein thelight-metal hydride is magnesium hydride, titanium hydride, a titaniumcarbonate or a titanium hydride.
 19. A metal body obtained by theprocess according to claim
 1. 20. The metal body according to claim 19,which is a component for a vehicle.
 21. The metal body according toclaim 19, wherein the metal in the component is an aluminum alloy. 22.The metal body according to claim 19, which has a surface which isclosed on all sides and a hollow structure in the interior.
 23. A metalbody which has a surface which is closed on all sides and a hollowstructure in the interior.
 24. A process for producing a metal foam in adie-casting machine comprising a sleeve or shot chamber and a die cavitywhich comprises foaming, in the die cavity, a metal melt and from about0.01 to about 10% by weight, based on the metal melt, and at least oneblowing agent, whereby the blowing agent is solid at room temperature.25. The process according to claim 24, wherein the amount of blowingagent is from 0.01 to 10% by weight, based upon metal melt.
 26. Theprocess according to claim 24, wherein the amount of blowing agent isfrom about 0.1 to about 10% by weight based upon metal melt.
 27. Theprocess according to claim 24, wherein the blowing agent is alight-metal hydride.
 28. The process according to claim 24, wherein thelight metal hydride is magnesium hydride, titanium hydride, a titaniumcarbonate or a titanium hydrate.
 29. The process according to claim 27,wherein the light-metal hydride is autocatalytically produced.
 30. Theprocess according to claim 24, wherein the metal melt is a light metal.31. The process according the claim 29, wherein the light metal isaluminum or an aluminum alloy.
 32. The process according to claim 24,wherein the mixture comprising the metal melt and the blowing agent isformed by introducing the metal melt and the blowing agent into the diecavity at the same time.
 33. The process according to claim 24, whereinthe mixture comprising the metal melt and the blowing agent is formedbefore introducing the mixture to the die cavity.
 34. The processaccording to claim 33, wherein the mixture comprising the metal melt andthe blowing agent is formed in the sleeve or shot chamber and is thenintroduced into the die cavity.
 35. The process according to claim 24,wherein the mixture comprising the metal melt and the blowing agent isformed by introducing first the metal melt into the die cavity and thenadding the blowing agent.
 36. The process according to claim 24, whereinthe mixture comprising the metal melt and the blowing agent is formed byfirst introducing the blowing agent into the die cavity and then addingthe metal melt.
 37. The process according to claim 24, wherein the diecavity is completely filled with the mixture comprising the metal meltand the blowing agent before foaming said mixture.
 38. The processaccording the claim 24, wherein the die cavity is underfilled by adefined volume of metal melt.
 39. The process according to claim 24,wherein the process is a hot-chamber process.
 40. The process accordingto claim 24, wherein the process is a cold-chamber process.
 41. Theprocess according to claim 24, wherein the metal is nonferrous or basemetal.
 42. The process according the claim 24, wherein the metal isselected from the group consisting of magnesium, calcium, aluminum,silicon, titanium zinc and an alloy of said metals.
 43. A metal bodyobtained by the process according to claim
 24. 44. The metal bodyaccording to claim 43, which is component for a vehicle.
 45. The metalbody according to claim 43, which has a surface that is closed on allsides and a hollow structure in the interior.